Paper machine dryer fabrics containing hollow monofilaments

ABSTRACT

A paper machine dryer fabric includes hollow thermoplastic monofilaments to replace at least a portion of the wefts, also known as cross-machine direction strands. Fabrics including such monofilaments may be either a woven fabric, or a spiral fabric. The deformable nature of the hollow monofilaments decreaes the air permeability of the fabric, and in the case of spiral fabrics, improves monofilaments retention within the helical coils between the hinge yarns. The hollow monofilaments have a solidity in the range of from about 60% to about 75%. A suitable thermoplastic is polyethylene terephthalate. Hollow monofilaments do not have the disadvantages of other deformable yarns, such as spun yarns, multifilament yarns or plied monofilament yarns, each of which tend to hold and entrap within their structure both water and foreign matter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a national stage application of PCT/CA93/00075, filed Feb. 25,1993, which is a continuation-in-part of U.S. application Ser. No.07/982,118, filed Nov. 25, 1992, now abandoned, which is a continuationof U.S. application Ser. No. 07/843,155, filed Feb. 28, 1992, nowabandoned.

FIELD OF THE INVENTION

This invention relates to fabrics intended for use in the manufacture ofpaper and like products, in which hollow monofilaments replace at leasta portion of the wefts, also known as cross-machine directions strands.The invention is particularly applicable to paper machine dryer fabrics.

DESCRIPTION OF THE PRIOR ART

The primary function of a dryer fabric is to hold the paper web incontact with the heated surfaces of the dryer cylinders. This increasesthe efficiency of heat transfer and improves the flatness of the paper.

An important property of dryer fabrics intended for use in modern,high-speed paper making machines is low permeability to air flow. Dryerfabrics must have low air permeabilities so as to prevent sheet flutterand, ultimately, breakage of the sheet (as documented by Race, Wheeldon,et al. in TAPPI, vol. 51, no. 7, July 1968). Low air permeability valuesmay be considered to be those in the range of 127 cm³ /cm² ·s (250 ft³/min/ft²) or below. It is also desirable that the air permeability ofthe fabric be constant throughout both the fabric itself, and itsoperational life.

It is difficult to obtain low air permeabilities in woven dryer fabricswhen solid monofilaments are used as the weft strands. Manufacturers ofdryer fabrics have thus traditionally resorted to incorporating spunyarns, multifilament yarns or plied monofilaments in order to obtain lowair permeabilities in conventional dryer fabric designs. These types ofyarns, however, make it difficult to accurately control fabric airpermeability during manufacture. They also allow foreign matter tobecome entrapped in the fabric, which changes the air permeability ofthe fabric throughout its life on the paper making machine. Trappedcontaminants are usually distributed unevenly in the fabric, and willcause uneven drying of the paper web. The use of spun yarns,multifilament yarns, or plied monofilaments in dryer fabrics alsoreduces the efficiency with which water is evaporated out of the paperweb, because water tends to condense and be retained within such yarns.

Another method of lowering fabric air permeability is to use machinedirection strands that are essentially rectangular in cross-section.Such a method is disclosed by Buchanan et al. in U.S. Pat. No.4,290,209. This patent also discloses the use of shaped or hollowmonofilaments as weft strands to further reduce dryer fabric airpermeability. However, it does not teach the critical physicalparameters required for the hollow monofilaments, such as stranddiameter, or solidity of cross-sectional area. No data is disclosed asto the effectiveness of hollow monofilament weft strands in reducingfabric air permeability.

Goetemann, et al., in U.S. Pat. No. 4,251,588 teach the use of hollowmonofilaments to improve dimensional stability and flex life in papermachine clothing. The range of void fractions in the yarncross-sectional area disclosed is from 0.03 to 0.15 (3% to 15%), or arange of solidities of from 97% to 85%. Solidities less than 85% werenot recommended because such monofilaments would flatten from a circularcross section to a void-free filament. Goetmann et al. also teach thatconventional techniques may be used to weave these hollow monofilamentsinto papermaking fabrics without collapsing them. No consideration isgiven to any interrelationship between the strand diameter of the hollowmonofilament, its solidity, and the space available within the wovenstructure to accommodate the yarns. The use of these hollowmonofilaments for the purpose of reducing fabric air permeability is nottaught.

It is also difficult to obtain low fabric air permeabilities in spiralfabrics. These fabrics are assembled from a multiplicity of helicalcoils which are intermeshed and connected together in a hingedrelationship by hinge yarns, substantially as described by Kerber in DE2,419,751, Leuvelink in U.S. Pat. No. 4,345,730, and Dawes in U.S. Pat.No. 4,481,079. The air permeability of these fabrics is typicallyaltered by inserting a shaped, solid monofilament into the space withinthe helical coils and between the hinge yarns. The cross-sectional shapeof the inserted monofilament is determined so as to efficiently fill thespace between the hinge yarns, thus lowering the air permeability of thefabric. Commonly used shapes include: ellipses, rectangles, trapezoids,a "D", or a "dog bone". It is also known to perforate such yarns alongtheir length so as to further assist in controlling air permeability, astaught by Gauthier in U.S. Pat. No. 4,567,077. However, a disadvantageof using shaped monofilaments in spiral fabrics is that they are noteffectively locked in position and are prone to falling out during thedrying operation on the paper making machine.

The predominant material used in the manufacture of dryer fabrics ispolyethylene terephthalate (PET) that has been stabilized to reduce itsrate of hydrolytic degradation. However, in the harshest dryer sections,where high temperatures (greater than 150° C.) occur, other moreexpensive polymers are commonly used. Such polymers include: blends of.polyphenylene sulphide (PPS), as disclosed by Baker et al. in U.S. Pat.No. 4,755,420, and polyetherether ketone (PEEK), as disclosed byDiTullio in U.S. Pat. No. 4,359,501 and Searfass in U.S. Pat. No.4,820,571. While vastly superior to PET in hydrolysis resistance, theirhigher cost restricts their usage due to economic considerations.

SUMMARY OF THE INVENTION

This invention seeks to overcome the aforementioned deficiencies of theprior art by providing a papermaker's heatset fabric, for use in papermaking or like machines, wherein at least a portion of the weft strandsare hollow thermoplastic monofilaments which have a solidity in theirundeformed cross-sectional area of from about 50% to about 80%, andwhich have a diameter such that they are deformed in the weft passagewayto be filled in the woven fabric during heatsetting.

Thus in a first broad embodiment, this invention seeks to provide awoven, heatset fabric, for use in papermaking and like machines, whereinat least a portion of the weft strands are hollow thermoplastic polymermonofilaments which have a solidity in their undeformed cross-sectionalarea of from about 50% to about 80%, and wherein the circumference ofsaid hollow monofilaments is greater than, or equal to, the perimeter ofthe weft passageway they are to occupy in the fabric after heatsetting.

In a second broad embodiment, this invention seeks to provide a heatsetspiral fabric, for use in papermaking and like machines, comprising aplurality of helical coils interconnected by hinge yarns, includinghollow monofilament weft strands having a solidity in their undeformedcross sectional area of from about 50% to about 80%, located within thehelical coils and between the hinge yarns, wherein the diameter ofhollow monofilaments is greater than the interior length of the minoraxis of the helical coils in the heatset fabric, and further wherein thehollow monofilaments are deformed by the helical coils as a consequenceof heatsetting of the fabric.

In either a woven or spiral fabric according to this invention, thehollow monofilaments will generally have an outside diameter in therange of between about 0.25 mm and 2.1 mm.

For the purposes of the present application, the following terms aredefined for use herein as shown:

Heatsetting: processes such as are well known to those skilled in theart whereby a fabric structure is stabilized under conditions ofelevated temperature and tension;

Perimeter of the weft passageway: the perimeter of the projection of thepassageway into which a weft yarn is to be placed, onto a plane which isnormal to the weft direction. It is understood that such a passagewaywill not have a constant or continuous cross-section in space along thelength of the weft strand, and therefore the hollow monofilament willnot be squeezed uniformly along its length at every warp intersection;

Solidity: the percentage of solid material which is present at anycross-section through the undeformed hollow monofilament prior toheatsetting, relative to the total cross-sectional area of themonofilament that is enclosed by its circumference at thatcross-section; and

Weft: cross-machine direction strands of a woven fabric, or strandswhich have been inserted into the helices and between the hinge yarns ofa spiral fabric.

Unless otherwise stated, all references made below to the diameter ofthe hollow monofilaments of this invention assume that thesemonofilaments have not been deformed in any way by heat setting.

The solidity of the hollow monofilaments intended for use in the papermachine fabrics of this invention is critical. We have found that theuseful range of solidities is from about 50% to about 80% with fromabout 55% to about 78% being preferable, and from about 60% to 75% mostpreferable. We have experimentally determined that solidities withinthis range will provide these monofilaments with adequate deformationalcapability, a critical factor in lowering the air permeability of afabric. If the solidity is too low, the hollow monofilaments mayfracture or deform excessively, or be destroyed during weaving. If thesolidity of the hollow monofilaments is too high, inadequate deformationoccurs and the resulting reduction in fabric air permeability will beinsignificant. This range of solidity also provides the monofilamentswith sufficient mechanical strength so as to withstand the rigours offabric creation, heat setting, seaming, assembly and subsequent use inthe paper machine.

The sizing of these hollow monofilaments is an important feature of thisinvention. We have discovered that the effectiveness of the hollowmonofilaments is greatest when their exterior circumference, prior toheatsetting, is greater than or equal to the perimeter of the weftpassageway they are to occupy in a woven fabric after heatsetting. Iftheir circumference is less than this value, then air permeability canonly be reduced by increasing the weft count (number of wefts per unitlength) of the fabric. This will reduce the perimeters of the weftpassageways in the cloth, thereby allowing the hollow monofilaments tonow fill the space between the warp yarns.

We have found that, for currently available fabrics, the usefulcircumference of the hollow monofilaments of this invention, for use inwoven fabrics, will correspond to diameters of from about 0.25 mm toabout 1.2 mm. Hollow monofilaments whose circumference corresponds todiameters of from about 0.50 mm to about 2.1 mm will be of use in spiralfabrics.

The outer diameter of a hollow monofilament that will completely fillthe perimeter of the available space in a heatset fabric is estimated bycalculating the perimeter of the shape to be filled, and equating thatvalue to the outer circumference of the hollow monofilament, hence itsoutside diameter, using the relation:

    C=πd.                                                   [Equation 1]

where

C=circumference, and d=diameter.

If the circumference of the hollow monofilament is selected so as to begreater than or equal to the perimeter of the weft passageway in theheatset fabric, the maximum solidity of the hollow monofilament whichwill not alter the geometry of the fabric should then be determined.Increasing the solidity beyond this maximum generally increases fabricthickness which, in turn, increases air permeability.

If the outer diameter of the monofilament is calculated using Equation1, and is equal to the perimeter of the area to be filled, then themaximum solidity can be calculated by assuming that all of the solidmaterial of the round hollow monofilament is deformed either elasticallyor plastically during weaving and heatsetting until the void space ofthe hollow monofilament is entirely consumed. The calculations whichfollow assume that the material is incompressible and that the fabric isheatset, unless indicated otherwise.

If the perimeter of the weft passageway to be filled is, for example, asquare whose sides are of a length a, then the perimeter C of the squareis:

    C=4a.                                                      [Equation 2]

Assuming the circumference of the hollow monofilament is equal to thisperimeter, then from Equation 1:

    C=4a=πd.                                                [Equation 3]

Solving for d,

    d=(4/π)a.                                               [Equation 4]

This is the minimum diameter of a hollow monofilament that will fill theavailable space.

Solidity is defined as:

    S=(A.sub.s /A.sub.T)×100,                            [Equation 5]

where

S=solidity of the hollow monofilament,

A_(s) =cross sectional area of the hollow monofilament that is occupiedby solid material, and

A_(T) =total cross sectional area bounded by the outside diameter of thehollow monofilament.

A_(s) cannot exceed the cross sectional area to be filled, thus themaximum A_(s) is equal to the cross sectional area to be filled, whenthe hollow monofilament is completely deformed to a void-free filament;hence:

    A.sub.s =a.sup.2                                           [Equation 6]

and the total cross sectional area of the hollow monofilament is

    A.sub.T =(π/4)d.sup.2.                                  [Equation 7]

Substituting and solving for solidity S:

    S=[a.sup.2 /(π/4)d.sup.2 ]×100                    [Equation 8]

Substituting d from Equation 4:

    S={[(π/4)d].sup.2 /[(π/4)d.sup.2 ]}×100=S=(π/4)×100

    S≈78.5%

The above calculation demonstrates that a hollow monofilament whosesolidity is greater than 78.5% must alter the geometry of the fabric ifit is also sized in its outside diameter so as to fill the perimeter ofa square opening. Use of hollow monofilament size and soliditycombinations which will alter fabric geometry are not recommended. Thesecalculations are therefore intended to:

1) guide the user in choosing the optimum outside diameter of the hollowmonofilament for a particular application, and

2) indicate the maximum solidity which can be used at that diameterwithout altering the geometry of the fabric.

It is well known that heatsetting reduces the perimeter of the weftpassageways in the fabric, and those skilled in this art will appreciatethat the size of these weft passageways after heatsetting cannot bemeasured beforehand. As a guide only, the effective size of the hollowmonofilaments for use in the fabrics of this invention may be estimatedby measuring the perimeter of the weft passageways in the fabric priorto heatsetting, and then sizing the hollow monofilaments so that theircircumference is greater than, or equal to, that perimeter. However,care must be taken to ensure that the solidity of the hollowmonofilaments is low enough so as not to alter the geometry of thefabric after heatsetting.

We have experimentally determined that the practical lower limit of thesolidity of these hollow monofilaments for paper machine fabricapplications is about 50%, and is controlled by two unexpected factors.

1) Hollow monofilaments with solidities below 50% tend to buckle andcollapse, rather than deform and take the shape of the perimeter theyare to occupy, thus rendering them ineffective. This is particularlytrue when the circumference of the monofilament is equal to or greaterthan the perimeter of the space to be filled in the heatset fabric.

2) Hollow monofilaments with solidities below 50% are prone to crushing,and is easily damaged in industrial looms.

The present invention seeks to provide a woven dryer fabric, for use inthe manufacture of paper and like products, whose air permeability isboth low and uniformly constant throughout. This objective is achievedin practice by incorporating hollow monofilaments of optimum stranddiameter and solidity as at least a portion of the fabric weft strands.

This invention also seeks to provide a spiral fabric, for use in thedryer section of paper making and like machines, whose air permeabilityis both low and uniformly constant throughout. This objective isachieved in practice by placing hollow monofilaments in the spacesbetween the hinge yarns within the helical coils of these fabrics,thereby eliminating any need to provide a specially shaped monofilament.The deformable nature of the hollow monofilaments improves theirretention within the spiral fabric during its operation on the papermaking machine, thus reducing the incidence of fabric failure due toloss of the solid, prior art yarns which had been "stuffed" into thesespaces.

Incorporation of hollow monofilaments in at least a portion of the weftpositions will provide the novel fabrics of this invention with thefollowing advantages over fabrics of the prior art:

1) a lower air permeability can be achieved while maintaining theall-monofilament characteristic of the fabric, with the resultingbenefits of cleaner operation;

2) less moisture is carried by the fabric;

3) a more consistent and uniform air permeability is provided throughoutthe fabric, because the physical characteristics of hollow monofilamentsare inherently less variable than those of spun yarns, multifilamentyarns or plied monofilaments; and

4) retention of spiral fabric weft under paper making machine operatingconditions is improved.

Further, the novel fabrics of this invention require less material, byweight, to manufacture than comparable prior art fabrics because thehollow monofilaments have less mass per unit length than solidmonofilaments of the same diameter. Their use is particularlyadvantageous when expensive polymers are required.

In addition, the weavability of paper machine fabrics can be improved byincorporating hollow monofilaments as at least a portion of the weftyarns. Since the hollow monofilaments have less mass than comparablysized solid monofilaments, their inertia is lower. This reduces problemsassociated with the acceleration and deceleration of large diametermonofilaments on high speed weaving looms, which, in turn, reducesweaving defects in the fabrics.

The incorporation of hollow monofilaments into paper making fabrics soas to reduce their air permeability is effective in both multi-layer andsingle layer fabric designs. A multi-layer fabric is one in which theweft strands lie in a series of essentially discrete tiers or planeswithin the fabric. A single layer fabric is one in which the weftstrands lie in essentially one common plane within the fabric.

Multi-layer fabrics, manufactured in accordance with the teachings ofthis invention, may contain hollow monofilaments selectively positionedin all layers, selected layers, or in only one layer, of a fabric.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described with reference to theexamples illustrated by the accompanying drawings in which:

FIG. 1 is a sectional view of an all-monofilament multi-layer dryerfabric of the prior art, in which all weft strands are solidmonofilaments;

FIG. 2 is a sectional view of a fabric substantially identical to thatshown in FIG. 1 in which the solid monofilament weft strands of theintermediate layer have been replaced with hollow monofilaments of theprior art, whose solidity is about 90%;

FIG. 3 is a sectional view of a fabric substantially identical to thatshown in FIG. 1, in which the solid monofilament weft strands of theintermediate layer have been replaced with hollow monofilaments whosesolidity is about 45%;

FIG. 4 is a sectional view of a fabric substantially identical to thatshown in FIG. 1, in which the solid monofilament weft strands of theintermediate layer have been replaced with hollow monofilament strandsaccording to the present invention;

FIG. 5 is an isometric view of a single layer, all monofilament dryerfabric in which 50% of the weft strands are hollow monofilamentsaccording to the invention;

FIG. 6 is a cross-section on the line I--I in FIG. 5;

FIG. 7 is a cross section on the line II--II in FIG. 5;

FIG. 8 is a sectional view of a spiral fabric into which hollowmonofilaments have been inserted according to the invention; and

FIG. 9 is a cross-section on line III--III of FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

Referring first to FIG. 1, there is shown diagrammatically theconstruction of an all-monofilament, 4-shaft, 12-repeat, multilayerdryer fabric of a design that is commonly used in the papermakingindustry. FIG. 1 illustrates the cross-sectional appearance of saidfabric following heatsetting. There are four consecutive warp strands,10, 11, 12 and 13. The weft strands comprise three layers. In sequencefrom the top of FIG. 1, these are strands 20, 21, 22, 23 and 24; in themiddle, strands 25, 26, 27 and 28; and at the bottom, strands 30, 31,32, 33 and 34. The intermediate layer of wefts, strands 25, 26, 27 and28, are solid monofilaments of the same diameter as the other wefts andare inserted into the fabric to assist in reducing its air permeability.It is known to use other yarns in this intermediate layer, such as spunyarns, plied monofilaments, or multifilaments.

A typical prior art fabric, made with the construction shown in FIG. 1,has an air permeability in the range of 152 to 203 cm³ /cm² ·s (300-400ft³ /min/ft²). Fabric air permeability is measured using the method andcalculations described in American Society for Testing and MaterialsStandard ASTM-D-73775-75 The air permeability figures given below weremeasured according to this method using a Frazier Air Permeometer.

FIG. 2 illustrates diagrammatically a heatset dryer fabric whose weavedesign is substantially identical to that shown in FIG. 1. This fabricdiffers from that shown in FIG. 1 in that hollow monofilaments of theprior art, having a solidity of about 90% and whose diameter issubstantially the same as the solid wefts, have been inserted in placeof the solid monofilaments in the intermediate layer. That is, wefts 1,2, 3 and 4, which are in the same place as wefts 25, 26, 27 and 28 inFIG. 1, are hollow monofilaments as taught by Goetmann et al.Accordingly, a cross-section taken through these high solidity strandsshows that they have undergone minimal deformation when woven into afabric and subsequently heatset. The physical properties of these priorart hollow monofilaments are so similar to those of comparably sizedsolid monofilaments, that the air permeability of a fabric in which theyare incorporated is not significantly reduced in comparison, forexample, to an identical, solid yarn fabric such as is shown in FIG. 1.

FIG. 3 illustrates diagrammatically a heatset dryer fabric whose weavedesign is also substantially identical to that shown in FIG. 1. Thesolid monofilament wefts, 25, 26, 27 and 28 in the intermediate layer ofFIG. 1, have now been replaced with hollow monofilaments 5, 6, 7 and 8whose solidity is approximately 45% and whose diameter is substantiallythe same as the solid wefts. A hollow monofilament having 45% solidity,will have a wall thickness of only some 26% of the monofilament radius.FIG. 3 is provided to illustrate the deformation which would occur tothese low solidity hollow monofilaments when incorporated into theintermediate weft positions. As can be seen, the relatively thin wallsof these monofilaments were crushed by the forces of weaving, and didnot deform so as to fill the available space in the desired manner.Thus, these low solidity monofilaments did not achieve the desiredeffect of consistently reducing air permeability throughout the fabric.

FIG. 4 illustrates diagrammatically a heatset dryer fabric manufacturedin accordance with the teachings of the present invention and whoseweave design is substantially identical to that shown in FIG. 1. Hollowmonofilament wefts 40, 41, 42 and 43, whose solidity is about 73% andwhose diameter is approximately 40% greater than that of the solid wefts25, 26, 27 and 28 in FIG. 1 they replace, have now been inserted in theintermediate layer of this fabric. It will be noted that the hollowmonofilaments have deformed upon heatsetting so as to fill the perimeterof the weft passageway, thereby effectively lowering fabric airpermeability in comparison to the similar fabrics of FIGS. 1, 2 and 3.

FIGS. 5, 6 and 7 illustrate diagrammatically a 4-shed, 4-repeat, singlelayer, heatset dryer fabric, substantially as taught in U.S. Pat. No.5,103,874 and which was woven in experimental trials. As is shown inthese Figures, the warp yarns are woven in pairs so as to position onemember of each warp yarn pair, 50 & 52, substantially above the other,51 & 53. Both yarns of a warp yarn pair, 50 & 51 and 52 & 53, then passtogether over the same side of each of the hollow monofilament weftyarns 61, 63 & 65. Upon heatsetting, the thicker, solid weft yarns 60,62 & 64 remain more or less straight, whilst the thinner, hollow wefts61, 63 & 65 are effectively deformed by warps 50 & 51 and 52 & 53passing around them, as is shown in FIG. 7, so as to substantially fillthe perimeter of the weft passageways, thereby lowering fabric airpermeability. The hollow monofilaments of this invention areparticularly useful when incorporated as at least a portion of the weftyarns in double warp, single layer fabrics such as are illustrated inFIG. 5.

FIG. 6 is a cross section taken at Line I--I in FIG. 5. As each warpyarn pair, 50 & 51 and 52 & 53, approaches a solid monofilament 64,their paths diverge so that one warp yarn pair member, 50 & 52, passesover solid weft 64, whilst the other warp yarn pair member, 51 & 53passes beneath. Solid monofilament 64 has not been deformed by anyappreciable amount during heatsetting so as to more effectively fill theperimeter of the weft passageway.

FIG. 7 is a cross-section taken at Line II--II in FIG. 5. This Figure isprovided to illustrate the deformation occurring when a hollowmonofilament, 61, that is oversized for this position in comparison to asolid weft, is used to fill the weft passageway. It will be noted thatthe hollow monofilament 61 is deformed during weaving and by theheatsetting process so as to more completely fill the perimeter of theweft passageway than would either a solid monofilament.

Table 1 displays the effects on fabric air permeability obtained byintroducing hollow monofilaments, as at least a portion of the weft,into both multi- and single-layer dryer fabrics, identified as Samples 1and 2, and Samples 3 and 4 respectively.

The multi-layer fabrics of FIGS. 1 and 4 were both woven in experimentaltrials, and are identified in Table 1 as Samples 1 and 2 respectively.Both Samples had nearly identical mesh counts, and were heatset underidentical conditions. The difference between Samples 1 and 2 is thatSample 2, in accordance with the teachings of this invention, containshollow monofilaments placed in one third of its weft positions. The 0.50mm solid monofilament wefts in the intermediate layer of Sample 1 werereplaced with 0.70 mm hollow monofilaments having a solidity of 73%.Comparing Samples 1 and 2, a reduction in fabric air permeability ofabout 49 cm³ /cm² ·s (96 ft³ /min/ft²) was achieved by replacingone-third of the solid wefts with hollow wefts of the present invention.

The data of Samples 3 and 4 in Table 1 was obtained from two 4-shed,4-repeat single layer dryer fabrics, substantially as shown in FIGS. 5,6 and 7, which were woven in experimental trials. In Sample 3, all ofthe weft yarns were solid monofilaments with diameters of 0.5 mm and 0.9mm and placed in alternating positions. In Sample 4, 0.7 mm diameterhollow monofilaments of 73% solidity replace every 0.5 mm solid weftyarn in Sample 3. Both Samples have substantially the same mesh countsand were heatset under identical conditions. Comparing Samples 3 and 4,it will be seen that a reduction in fabric air permeability of 46 cm³/cm² ·s (90 ft³ /min/ft²) was achieved by replacing one-half of thesolid wefts of Sample 3 with hollow wefts according to the presentinvention, as in Sample 4.

                  TABLE 1    ______________________________________    Effect of Hollow Monofilaments on Dryer    Fabric Air Permeability    ______________________________________                     Sample 1  Sample 2    ______________________________________    Mesh (cm.sup.-1) (a)                     16.9 × 19.5                               16.9 × 18.7    Solid Monofilament                     0.5       0.5    Size (mm)    % Solid Wefts    100       67    Hollow Monofilament                     n/a       0.7    Size (mm)    % Hollow Wefts    0        33    Hollow Weft      n/a       73    Solidity (%)    Air Permeability 176       127    (cm.sup.3 /cm.sup.2 · s) (b)    Difference in Dryer Fabric Air Permeability    (Sample 1 - Sample 2) = 49 cm.sup.3 /cm.sup.2 · s    ______________________________________                     Sample 3  Sample 4    ______________________________________    Mesh (cm.sup.-1) (a)                     22.4 × 1.5                               22.8 × 7.5    Solid Monofilament                     0.9 & 0.5 0.9    Size (mm)    % Solid Wefts    100       50    Hollow Monofilament                     n/a       0.7    Size (mm)    % Hollow Wefts    0        50    Hollow Weft      n/a       73    Solidity (%)    Air Permeability  84       38    (cm.sup.3 /cm.sup.2 · s) (b)    Difference in Dryer Fabric Air Permeability    (Sample 3 - Sample 4) = 46 cm.sup.3 /cm.sup.2 · s    ______________________________________     NOTES:     (a) mesh count = number of Warps per cm × number of Wefts per cm     (b) air permeability as measured by test method ASTMD-737-15.

Table 1 shows that, under equivalent manufacturing conditions, asubstantial reduction in air permeability is achieved by theintroduction of hollow weft, which fill more completely the weftpassageway than the solid weft they replace, as a portion of the crossmachine direction strands.

A hollow monofilament, whose size and solidity are determined inaccordance with the teachings of this invention, will effectivelyreplace a solid monofilament in various fabric designs. This is becausesuch a hollow monofilament is more readily deformable and will fill theavailable space in the fabric more effectively than a solid, andrelatively unmalleable, monofilament. This deformation will allow afabric to attain a lower air permeability than a comparable fabric,containing either solid monofilaments in the same positions andmanufactured under equivalent conditions, or one containing hollowmonofilaments whose size and solidity are not selected according to thecriteria provided herein.

All of the solid monofilament weft yarns in a woven fabric can bereplaced with hollow monofilament yarns. Table 2 shows data obtained byreplacing all the solid monofilament wefts in a multilayer fabric withslightly larger hollow monofilaments. Both woven samples have nearlyidentical mesh counts, and were heatset under identical conditions. InSample 6, 0.55 mm hollow monofilaments replace all of the 0.40 mm solidmonofilaments of Sample 5. A reduction in fabric air permeability ofabout 23 cm³ /cm² ·s (45 ft³ /min/ft²) was achieved in Sample 6 overSample 5.

                  TABLE 2    ______________________________________    Effect on Fabric Air Permeability obtained by    Replacing all Solid Monofilaments with Hollow Monofilaments                   Sample 5                           Sample 6    ______________________________________    Mesh (cm.sup.-1) 20.3 × 21.1                               20.3 × 17.7    Solid Monofilament                     0.40      n/a    Size, mm    % Solid Wefts    100       Zero.    Hollow Monofilament                     n/a       0.55    Size, mm    % Hollow Wefts   Zero      100    Hollow Weft      n/a       73    solidity, %    Air Permeability,                      66       43    cm.sup.3 /cm.sup.2 · s)    ______________________________________     Difference in air permeability, Sample 5 - Sample 6: 23 cm.sup.3 /cm.sup.     · s.

FIGS. 8 and 9 illustrate a diagrammatically spiral fabric into whichhollow monofilaments have been inserted within the helical coils andbetween the hinge yarns. In this form of dryer fabric, a sequence ofhelical coils, as at 70, 71, 72, in which the axes of the helices are inthe weft direction, are joined together by inserted hinge yarns as at 73and 74, which are also in the weft direction. In this example, thehelical coils adopt a flattened, somewhat oval, configuration afterheatsetting, as is shown in FIG. 8. The length of the minor axis of theinternal void area of the helical coil is labelled "h".

The internal void volume between adjacent areas of the helical coils ofsuch a fabric, as at 75 and 76, is free space and contributes directlyto the air permeability of the fabric. As shown in FIG. 8, a hollowmonofilament as at 77, 78 and 79, whose outside diameter is greater thanor equal to the length h of the minor axis of the helical coils afterheatsetting, has been inserted into the middle of the joined helicalcoils during fabric construction. When the fabric is heatset, the lengthh of the minor axis of the helical coil is reduced and the hollowmonofilament is deformed into a somewhat oval shape, effectively andefficiently filling the internal void volume within the coil, as shownat 78, so as to decrease fabric air permeability.

We have found that hollow monofilaments are most effective in thisposition when their outside diameter, prior to heatsetting, is equal toor greater than the length, h, of the minor axis of the heatset coilinto which they have been inserted. This causes the monofilaments todeform during heatsetting, which serves to hold them in place andprevents the yarns from falling out of the fabric during its life on thepaper machine. This deformation of the hollow monofilament in a spiralfabric can be seen in the cross-section parallel to the axis of thespiral shown in FIG. 9.

As previously noted, the useful range of hollow monofilament soliditiesof this invention is from about 50% to about 80%, and is preferably fromabout 55% to about 78%, and is most preferably from about 60% to about75%. We have found that this range of solidities is also critical tospiral fabrics because it provides the hollow monofilaments with:

a) sufficient stiffness to allow them to be inserted into the helicalcoils and between the hinge yarns by methods currently known in themanufacture of spiral fabrics, and

b) sufficient malleability to allow them to deform during furtherprocessing, so as to fill the interstitial spaces within the helicalcoils and between the hinge yarns; this deformability is the criticalfactor in lowering fabric air permeability.

Table 3 displays data relating to spiral fabrics which have beenassembled using helices made entirely of PET, and into which both solidand hollow monofilaments also made from PET have been inserted into thespaces within the helical coils and between the hinge yarns. All sampleswere manufactured and heatset under identical conditions. Sample A doesnot contain any yarns inserted into this position, and therefore acts asa control. So-called "dog-bone" shaped solid monofilaments have beeninserted into this same position in Sample B. Samples C-F contain hollowmonofilaments of progressively greater diameters and varying soliditiesinserted into the spaces within the helical coils and between the hingeyarns. The number of spirals per centimeter of cross machine direction(spiral count), hinge yarns per centimeter of machine direction (yarncount), and the hinge yarn diameter, are the same for all samples.

As can be seen from Table 3, a significant reduction in fabric airpermeability is achieved by inserting hollow monofilaments, whosediameter, prior to heatsetting, is from 1.8 mm to 2.1 mm, into thespaces within the helical coils and between. the hinge yarns. The bottomrow of Table 3, labelled "Air Permeability Net Change", shows the netdifference in air permeability obtained from each sample in comparisonto the control, Sample A. For example, the air permeability of Sample Chas been reduced by 252 cm³ /cm² ·sec (495 ft³ /min/ft²) in comparisonto the control by the insertion of 1.8 mm hollow monofilaments.Similarly, the air permeability of Samples D and E have been reduced by276 cm³ /cm² ·sec (542 ft³ /min/ft²) and 312 cm³ /cm² ·sec (613 ft³/min/ft²) respectively by insertion of 1.9 mm and 2.0 mm diameter hollowmonofilaments. A net change in air permeability of 332 cm³ /cm² ·sec(652 ft³ /min/ft²) in comparison to the control is realized when alarger, 2.1 mm diameter hollow monofilament, is inserted into the sameposition, as in Sample F.

                  TABLE 3    ______________________________________    Effect on Spiral Fabric Air Permeability obtained    by Inserting Hollow Monofilaments made from PET             SAMPLE NO    Parameter  A      B        C    D     E    F    ______________________________________    Spiral Count               6.5    6.5      6.5  6.5   6.5  6.5    (cm.sup.-1)    Hinge Yarn 2.4    2.4      2.4  2.4   2.4  2.4    Count (cm.sup.-1)    Hinge Yarn  0.10   0.70     0.70                                     0.70  0.70                                                0.70    Diameter (mm)    Inserted Weft               n/a    0.45 ×                               1.8  1.9   2.0  2.1    Size (mm)         2.2    Inserted Weft               n/a    100      63.4 74.2  65.9 66.5    Solidity (%)    Fabric Air 432    196      180  156   120  100    Permeability    (cm.sup.3 /cm.sup.2 · sec)    Fabric Air  0     236      252  276   312  332    Permeability    Net Change    ______________________________________

The data displayed in Table 3 shows that, in general, as the unheatsetsolidity and diameter of the hollow monofilaments increase together,heatset fabric air permeability values decrease. We have found that theoptimum range of solidity of hollow monofilaments is from about 50% toabout 80%, with from about 55% to about 78% being more effective, whilstsolidities of from about 60% to about 75% are most effective in reducingfabric air permeability. We have also found that the effective diameterof the inserted hollow monofilaments prior to heatsetting will be afunction of the length h of the minor axis of the heatset helical coilsinto which they have been inserted, and this diameter should be equalto, and is preferably greater than, the length h of the minor axis ofthe heatset helical coil.

Table 4 displays data obtained from PET spiral fabrics into which hollowmonofilaments made from polybutylene terephthalate (PBT), or a blend of10% HYTREL® in PET, have been inserted into the spaces within thehelical coils and between the hinge yarns. Fabric Samples G and Hcontain hollow monofilaments made from PBT, and Samples J, K and Lcontain hollow monofilaments extruded from a blend of 10% HYTREL® inPET. The design of the fabric samples used to obtain this data issubstantially identical to that used in the samples of Table 3 and allwere manufactured and heatset under identical conditions. All airpermeability net changes are again made in comparison to the control,Sample A, which is the same control used in Table 3. HYTREL® is aregistered trademark of DuPont and is a polyester elastomer.

                  TABLE 4    ______________________________________    Effect on Spiral Fabric Air Permeability Obtained    by Inserting Hollow Monofilaments made from PBT    or 10% HYTREL in PET             SAMPLE NO.    Parameter  A       G      H     J     K    L    ______________________________________    Spiral Count               6.5     6.5    6.5   6.5   6.5  6.5    (cm.sup.-1)    Hinge Yarn 2.4     2.4    2.4   2.4   2.4  2.4    Count (cm.sup.-1)    Hinge Yarn  0.70    0.70   0.70  0.70  0.70                                                0.70    Diameter (mm)    Inserted Weft               n/a     2.0    2.1   1.7   1.9  2.0    Diameter (mm)    Inserted Weft               n/a     56.3   60.4  71.9  72.6 72.0    Solidity (%)    Fabric Air 432     199    140   269   232  156    Permeability    (cm.sup.3 /cm.sup.2 · sec)    Fabric Air  0      233    292   163   200  276    Permeability    Nat Change    ______________________________________

The data provided in Table 4 shows that the hollow PBT monofilaments ofSamples G and H, and the hollow yarns made from 10% HYTREL in PET ofSamples J, K and L, were both effective upon heatsetting in reducingfabric air permeability. Table 4 shows that it is possible to obtain netreductions in fabric air permeability which are similar to thoseobtained using hollow PET monofilaments by using other polymers underequivalent manufacturing conditions. The data displayed in Tables 3 and4 indicate that hollow monofilaments made from PET are the mosteffective in reducing spiral fabric air permeability, while the polymerblend of 10% HYTREL® in PET is less effective, and PBT is the leasteffective among the polymers tested.

Although the selection of the polymer from which the hollowmonofilaments are made will have an impact on the effectiveness of thesestrands in reducing fabric air permeability, we have found that varyingthe solidity of the strands is the most effective means of decreasingfabric air permeability. Thermoplastic polymers other than PET, PBT, andblends thereof, may be found which will provide hollow monofilamentswhose physical properties and characteristics would make them successfulcandidates for use in the fabrics of this invention. Polyphenylenesulphide (PPS) and polyetherether ketone (PEEK) are examples of suchpolymers, but the invention is not limited to the polymers referencedherein. In experimental and field trials to date, we have found that PETis an effective polymer for these applications.

As previously noted, the useful diameter of the hollow monofilamentsintended for use in woven fabrics will generally be in the range of fromabout 0.25 mm to about 1.2 mm, while spiral fabrics will utilize yarnswhose diameter is from about 0.50 mm to about 2.1 mm. The most effectivestrand diameter for a particular application will be a function of theavailable space in the fabric: in a woven fabric, the circumference ofthe strand will ideally be greater than or equal to the perimeter of theweft passageway in the heatset fabric into which it will be placed,whilst in a spiral fabric, the strand diameter will ideally be greaterthan the interior length of the minor axis of the heatset spiral.

A significant portion of the expense of manufacturing dryer fabrics isthe cost of the material used. By replacing at least a portion of thesolid monofilament wefts with hollow monofilaments of the same diameterin a dryer fabric, the mass of material used per unit area of fabric canbe reduced, and a reduction in material costs can be realized. This isparticularly important when expensive polymers, such as PPS and PEEK,are used to make the monofilaments.

Wide industrial looms, up to 15 meters in width, are used in themanufacture of dryer fabrics. The requirement to traverse suchdistances, in a minimum of time, with a shuttle that carries the weftstrands, demands high levels of acceleration and deceleration of boththe shuttle and the strand at each side of the loom. The weft strandsused in modern dryer fabric designs, particularly single layer designs,can be relatively massive (from about 0.7 mm to about 1.2 mm indiameter). The inertial effects associated with the acceleration anddeceleration of these large wefts can cause difficulties in weaving,resulting in fabric defects and lowered production.

For example, the monofilament can pull out of the shuttle uponacceleration, and thus not be carried across the entire width of theloom, creating a defect in the fabric called a "dropped weft". Ondeceleration of the shuttle at the opposite side of the loom, themonofilament can continue to traverse the loom after the shuttle hasstopped, thus providing a length of monofilament that is greater thanthe width of the loom. On beat-up into the fabric, the excess length ofmonofilament is trapped in the fabric, creating a defect called a"double weft". One method of reducing defects such as these, which arecaused by inertial effects, is to reduce the mass of the solidmonofilament used as the weft strand by replacing it with a hollowmonofilament of substantially the same overall diameter.

Other embodiments of the invention may be made using the principlesclaimed herein. The specific embodiments should not be considered aslimitations of the invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A heatset fabric,including weft strands, for use in papermaking machines, wherein atleast a portion of the weft strands are hollow thermoplastic polymermonofilaments which, prior to heat setting, have a solidity in theirundeformed cross-sectional area of from about 60% to about 75%, andwherein the circumference of said hollow monofilaments is greater than,or equal to, the perimeter of the weft passageway they are to occupy inthe fabric after heatsetting.
 2. A woven, heatset fabric, for use inpaper making machines, wherein at least a portion of the weft strandsare hollow thermoplastic monofilaments which, prior to heat setting,have a solidity in their undeformed cross-sectional area of from about60% to about 75% and wherein the circumference of said hollowmonofilaments is greater than, or equal to, the perimeter of the weftpassageway they are to occupy in the fabric after heatsetting.
 3. Aheatset spiral fabric, for use in papermaking machines, comprising aplurality of helical coils interconnected by hinge yarns, includinghollow monofilament weft yarns which, prior to heat setting, have asolidity in their undeformed cross sectional area of from about 60% toabout 75%, located within the helical coils and between the hinge yarns,wherein the diameter of said hollow monofilaments is greater than theinterior length of the minor axis of the helical coils in the heat setfabric, and further wherein the hollow monofilaments are deformed by thehelical coils as a consequence of heatsetting the fabric.